Reduction processes with moving bed reactors are widely known in the art. Generally, they comprise two zones, the first, in the upper part of the reactor, is the so-called reduction zone where iron ore flowing downwardly by gravity and a stream of upwardly flowing high temperature reducing gases are contacted countercurrently, said reducing gases being gas mixtures largely composed of H.sub.2 and CO. In this zone preheating and reduction of iron ore are carried out.
In the second zone, at the lower part of the reactor, is the so-called cooling zone, where the descending hot and reduced iron ore particles are contacted countercurrently with an ascending flow of cold gas in order to cool the reduced iron ore particles before being discharged to the atmosphere. This cooling is necessary to avoid the reoxidation of the reduced particles with the oxygen present in the air.
The productivity of the reduction zone is determined by the time needed to reduce the iron ore particles, the smaller the residence time the greater the production that is achieved by the same reduction zone.
It is known that the higher the temperature of the reducing gas at the inlet of the reduction zone, the smaller the residence time of the solids in this zone. The above happens because the kinetics of the iron ore reduction reactions with H.sub.2 and CO depends strongly on the temperature. The higher the temperature, the faster the rate of reaction, and the higher the productivity of the process.
Usually direct reduction processes operate at a temperature between 750.degree. and 900.degree. C. at the inlet of the reduction zone.
The main limitation for further increasing this temperature is the tendency of sinterization and agglomeration shown by most of the highly reduced iron ores when they reach temperatures higher than 900.degree. C.
This limitation is particularly strong when dealing with iron ore particles rich in iron, especially in the form of pellets, because pellets have a high iron content and a low gangue content.
Nowadays, it is preferred to use pellets with a high iron content as feedstock for direct reduction processes. The main reason is that the pellets are, in general, more easily reduced than lump ores. This quality helps in obtaining a highly metallized product. Additionally, pellets are also more resistant to mechanical degradation during the reduction process and for this reason, they generate fewer fines than lump ores. It is also possible to vary, within certain limits, the chemical composition of the gangue in order to optimize the use of reduced material as a feedstock for electric arc furnaces.
These days the tendency in the iron and steel industry is to use pellets with an iron content higher than 67%. This aggravates the agglomeration problem, since it is known that if the iron content is higher, the pellets sinterization and agglomeration problems are greater.
When solids agglomeration happens in moving bed reactors, serious problems of solids flow and gas flow distribution are encountered. This causes loss in process control and erratic product quality.
Several solutions for solving the problem of agglomeration in moving bed reactors for the direct reduction of iron ores have been proposed. The most obvious is the use of mechanisms that destroy said agglomerates. This is a non-optimized solution, since these mechanisms are usually located in the path of solids flow causing disturbances to said flow, aggravating the problem. They are also subject to severe conditions of abrasion and high temperature. These mechanisms are complex and expensive.
Another known manner of solving the problem of pellets agglomeration when operating at high temperatures, is to charge the reactor with mixtures of pellets and lumps or pellets and an inert material of irregular shape. In both cases the shape effect is present which helps to minimize the problem of agglomerates.
In the case of lump ores there is the disadvantage that generally lumps are less reducible than pellets and also produce a greater quantity of fines. Additionally, there are few lump ores in the world that can be utilized in direct reduction processes. For this reason it is not always convenient to design the operation of direct reduction plants on the basis of using mixtures of pellets and lumps.
The disadvantage of using mixtures of inert materials and pellets is the need of separating the inert material from the product and the decrease in reactor productivity.
Due to the advantages of using pellets, e.g. high reducibility, low gangue, and lower fines generation; there is the need for a direct reduction process which consistently permits its operation with 100% pellets having a high content of iron, higher than 67%, at reduction temperatures above 900.degree. C., without problems of sintering and agglomeration.
U.S. Pat. No. 4,268,303 discloses a direct reduction process which permits operation at high temperatures without problems of agglomeration. The process disclosed in this patent is based on a moving bed reactor having two reduction zones without a cooling zone.
In the first zone, the reduction takes place at temperatures in the order of 950.degree. to 1200.degree. C. with gases having a high methane content (15-40%).
According to the teachings in this patent, it is possible to carry out the first reduction stages (30 to 80%) at high temperatures and when the methane content is high, because the reduction reaction of methane is highly endothermic.
In the second zone, the reduction is carried out at temperatures in the range between 750.degree. and 950.degree. C. with gases having a lower methane content (2-7%).
The principal limitation of this process is the extreme level to which the temperature of the gases with high methane content must be raised in order to carry out the reduction. On one hand, the materials needed for operating heaters at temperatures in the order of 1200.degree. C. are very specialized and expensive, and on the other hand, at these temperatures pyrolisis of methane is favored (causing problems of high carbon deposition which translate to operating problems of the reactor).
In this patent the high agglomerating tendency of pellets having high iron content is not mentioned nor is any way disclosed for solving this problem.
The present invention discloses a process based on a moving bed reactor having three zones, a reduction zone in the upper part of the reactor, a cooling zone in the lower portion of the reactor and an intermediate zone separating the above mentioned zones.
In the reduction zone, the reduction is carried out at temperatures in the order of 950.degree. C. with a gas having a content of methane between 4 and 10%, of hydrogen between 60 and 70%, and of carbon monoxide between 2 and 15%.
In the lower part of the reactor, the product cooling zone is located. Said cooling is effected in a closed loop comprising said lower part of the reactor, a quench cooler and a compressor. A stream of natural gas, mainly composed of methane, serves as make-up to this loop. Since there is no gas outlet, external to the reactor, in this cooling loop; the methane injected to said loop causes methane to flow therefrom through the intermediate zone to the reduction zone.
In the intermediate zone the methane coming from the cooling zone is mixed with a portion of the hot reducing gas injected to the reduction zone.
The cooling gas flowing from the cooling zone has a temperature between 400.degree. and 600.degree. C. When the cooling gas is contacted in the intermediate zone with the oxidant elements present in the hot reducing gas; the highly endothermic reforming reactions of methane are promoted. Due to these reactions, the temperature of the solids decreases rapidly, because the heat of reaction is provided by the descending mass of solids. This sudden cooling of solids avoids agglomeration of the highly metallized pellets and particles, because the time during which they are at high temperatures is very short.
In this way the agglomeration of particles of highly metallized pellets is avoided, without the need of having a high methane content in the reducing gas which implies a reduction zone at extreme temperatures (1200.degree. C.).
In the present invention the reduction takes place in a single stage with a mixture of hydrogen and carbon monoxide, said mixture having a reduction velocity higher than that of methane.
The reforming that takes place in the intermediate zone avoids the formation of agglomerates and makes possible a decrease in capacity of the natural gas reforming unit. U.S. Pat. Nos. 4,046,557 and 4,049,440 disclose natural gas injection to the cooling loop of a reduction process with a moving bed reactor. Nevertheless, the natural gas injection is always carried out with a supplementary injection of recirculated cooled reducing gas. The main object of the aforementioned patent is to utilize recirculated gas from the reduction loop as a cooling gas without affecting said reduction loop. The natural gas is injected in order to regenerate the reducing potential of the recirculated gas by reforming the natural gas in the cooling loop and then permitting a portion of this gas to upflow to the reduction loop. In the U.S. Pat. Nos. 4,046,557 and 4,049,440 the amount of methane injected to the cooling zone and then reformed in the reactor does not contribute to decrease the reforming unit capacity, because the amount of hot reformed gas flowing from said reformer is fixed by the temperature requirements at the reduction zone inlet. This temperature is fixed by the mix of the hot reducing gas with the cool recirculated gas. It is not possible to decrease too much the hot gas flow coming from the reforming unit without lowering the temperature at the reduction zone inlet. Thus, injecting natural gas to the cooling loop does not make it possible as a practical matter to decrease the reforming unit capacity. In the process according to the present invention, the reformed gas is injected cool as make-up to the reduction loop and the make-up mixture with the recirculating gas are heated before its injection to the reduction zone of the reactor. In this case the natural gas injection does help to reduce the size of the reforming unit.